Many of the world’s largest river deltas are sinking at alarming rates, putting over 236 million people at increased flood risk from subsidence alone, researchers have found.
River deltas are home to an estimated 350 to 500 million people globally, including 10 of the world’s 34 megacities. Rising seas pose a clear danger to these dynamic landscapes, but lack of high-resolution elevation-change measurements have long obscured the threat of subsidence, or sinking land.
A new analysis of a decades’ worth of satellite data across 40 major river deltas, reported by SciTechDaily, has revealed that in nearly all of these densely populated areas some portion is sinking faster than sea levels are rising. Average subsidence already outpaces local sea-level rise in 18 of the 40 deltas, increasing near-term flood risk for more than 236 million people.
Researchers used advanced satellite radar technology to create the first high-resolution, delta-wide maps of elevation loss across deltas in five continents, each pixel corresponding to 75 square metres of surface. More than simply showing where subsidence is occurring, the maps also quantify the rate, with some regions sinking at more than twice the current global rate of sea-level rise.
[See more: Thailand could lose its capital to climate change]
“Our results show that subsidence isn’t a distant future problem — it is happening now, at scales that exceed climate-driven sea-level rise in many deltas,” Manoochehr Shirzaei, co-author and director of Virginia Tech’s Earth Observation and Innovation Lab, said in a press release.
Among the deltas experiencing the most concerning rates of subsidence are the Mekong (Vietnam), Nile (Egypt), Chao Phraya (Thailand), Ganges-Brahmaputra (India), Mississippi (US), and Yellow River (China) systems. Sinking deltas bring not only increased flood exposure, but more destructive storm surges and contamination of drinking water sources by saltwater. Some populations may even be forced to relocate, particularly from low-lying areas.
“In many places, groundwater extraction, sediment starvation, and rapid urbanisation are causing land to sink much faster than previously recognised,” explained lead author Leonard Ohenhen, an assistant professor at the University of California, Irvine. Groundwater depletion has emerged as the strongest overall predictor of delta sinking, although there was regional variation.
“When groundwater is over-pumped or sediments fail to reach the coast, the land surface drops,” explained Susanna Werth, the Virginia Tech geoscientist who co-led the groundwater analysis. “These processes are directly linked to human decisions, which means the solutions also lie within our control.”
